A thin film transistor using an oxide semiconductor, such as indium gallium zinc oxide (IGZO) and indium tin zinc oxide (ITZO), as active layer, has a higher electron mobility; whereas a thin film transistor using low-resistivity metal materials such as copper (Cu) and silver (Ag) as gate, source and drain may have an substantially lower resistance; therefore, a thin film transistor in a large-size display device generally uses an oxide semiconductor as an active layer, and uses copper (Cu), silver (Ag) or other low-resistivity metal materials as gate, source and drain. Based on the above two advantages, the thin film transistor as manufactured can achieve a higher resolution, and a higher refresh rate, thus obtaining a better displaying effect.
FIG. 1 is a schematic diagram of a thin film transistor in a prior art large-size display device. As shown in FIG. 1, a thin film transistor 1 is manufactured on a substrate 2, and comprises a gate 10, an active layer 11, a source 12 and a drain 13. Therein, the active layer 11 uses an oxide semiconductor material such as IGZO, ITZO, etc. so as to make the thin film transistor 1 have a higher electron mobility; and the active layer 11 is located above the source 12 and the drain 13 (i.e. the source 12 and the drain 13 are manufactured prior to the active layer 11). The source 12 comprises a source first conductive layer 120 and a source second buffer layer 121 arranged below the source first conductive layer 120, and a source first buffer layer 122 arranged above the source first conductive layer 120. The drain 13 comprises a drain first conductive layer 130 and a drain second buffer layer 131 arranged below the drain first conductive layer 130, and a drain first buffer layer 132 arranged above the drain first conductive layer 130.
Specifically, the source first conductive layer 120 can be specifically made of Cu, to lower the resistance of the source. However, since Cu has a poor adhesion to the base layer where the source 12 resides (located below the source 12; when the thin film transistor 1 is a bottom gate structure, the base layer is generally a gate insulating layer, and when the thin film transistor 1 is a top gate structure, the base layer is generally the substrate 2), the source second buffer layer 121 is arranged below the source first conductive layer 120. The source second buffer layer 121 can be specifically made of molybdenum (Mo), titanium (Ti), chromium (Cr), molybdenum niobium alloy (MoNb), etc., in order to enable a greater adhesion between the source 12 and the base layer (the metal or alloy of Mo, Ti, Cr and MoNb have a greater adhesion to the base layer, while Cu has also a greater adhesion to these metals or alloy). The source first buffer layer 122 can also be made of Mo, Ti, Cr, MoNb, etc., for avoiding contact between an upper surface of the source first conductive layer 120 and the active layer 11, and thus preventing the Cu atoms from diffusing into the active layer 11 and preventing the oxygen in the active layer 11 from being absorbed by the source first conductive layer 120. Meanwhile, the source first buffer layer 122 can further form a fine ohmic contact with the active layer 11.
The drain first conductive layer 130, the drain second buffer layer 131, and the drain first buffer layer 132 in the drain 13 are similar to those in the source 12, which will not be repeated here.
In the above thin film transistor 1, a side wall of the source first conductive layer 120 and the drain first conductive layer 130 is in contact with the active layer 11, such that the easily diffusing Cu atoms in the source first conductive layer 120 and the drain first conductive layer 130 will diffuse into the active layer 11. Meanwhile, the source first conductive layer 120 and the drain first conductive layer 130 will also absorb the oxygen in the active layer 11, making the composition of the active layer 11 change, affecting the electron mobility of the thin film transistor 1, and causing the electric property and stability of the thin film transistor 1 to reduce.